1. Field of the Invention
This invention relates to a process for producing a flexible printed base having a good heat resistance and a superior folding endurance.
2. Description of the Related Art
Flexible printed bases are used for producing printed circuits having flexibility, and in recent years, it has been more and more required to make the size of electronic equipment smaller, make the base thinner and make the density of the circuits higher.
Conventional polyimide flexible bases have been mainly produced by applying a polyimide film onto a copper foil by the medium of an adhesive. However, since such bases use an adhesive, problems have been raised in the aspect of heat resistance, electric characteristics, etc.; hence such bases have been impossible to sufficiently make use of the characteristics thereof.
In order to solve these problems, a process of subjecting a polyimide film to hot-melt adhesion onto a copper foil has been disclosed in Japanese patent application laid-open No. Sho 57-181,857/1982 and a process of directly coating a copper foil with a polyimide precursor, followed by heating and drying treatment (hereinafter referred to as "direct coating process") is disclosed in Japanese patent publication Nos. Sho 61-111,359/1986 application laid-open and Sho 63-69,634/1988.
As to these improvement processes, since no adhesive is used, the heat resistance, electric characteristics, etc. are improved, but since the former process employs steps of production of film, its hot-melt adhesion, etc., it requires the same number of steps or more as that of conventional products (using adhesives). Whereas, according to the direct coating process, since it has no step of film production, simplicity of steps is possible and also the heat resistance and electric characteristics of the resulting flexible printed base are superior, but the process cannot contribute to the high folding endurance of the base. Further, for use applications requiring a high folding endurance, rolled copper foil rather than electrolytic copper foil has often been used as copper foil. However, in general, rolled copper foil is not only more expensive than electrolytic copper foil, but also the peel strength of the resulting flexible printed base lowers.
The present invention proposes a process for producing a cheap flexible printed base having solved the above problem and having a superior folding endurance.
It has been disclosed that when a flexible printed base is heat-treated at 100.degree. to 200.degree. C., its folding endurance is generally improved by about 1.5 times (Japanese patent application laid-open Nos. Sho 54-110,466/1979 and Sho 53-17,764/1978), but when an electrolytic copper foil is used for the base, the folding endurance does not reach that of rolled copper foil and is insufficient.
Further, when the electrolytic copper foil was subjected to heat curing in the atmosphere at 200.degree. to 450.degree. C, the folding endurance lowered. Whereas, when the above heat curing was carried out in an inert gas having an oxygen concentration of 0.5% or lower, preferably 0.2% or lower and under a tension of 0.02 to 0.2 Kg/cm, it has been found surprisingly enough that the folding endurance of the resulting flexible printed base was improved up to a similar folding endurance to that of rolled copper foil.
This improvement in the folding endurance has been found from X-ray diffraction pattern of the copper foil to originate from inhibition of thermal cleavage reaction of high molecules in the above-mentioned atmosphere and at the same time the crystalline configuration of the copper foil.
The present inventors have made extensive research on the improvement in the folding endurance of the printed base obtained according to the above direct coating process, and as a result have found that the folding endurance is improved by optimizing the conditions of the atmosphere and tension at the time of curing and this fact is greatly related to the crystalline size of copper of copper foil and the selective orientability of the crystalline surface of copper of copper foil.